Function esp_idf_bindgen::rtc_clk_cal

pub unsafe extern "C" fn rtc_clk_cal(
    cal_clk: rtc_cal_sel_t, 
    slow_clk_cycles: u32
) -> u32

@brief Measure RTC slow clock's period, based on main XTAL frequency

This function will time out and return 0 if the time for the given number of cycles to be counted exceeds the expected time twice. This may happen if 32k XTAL is being calibrated, but the oscillator has not started up (due to incorrect loading capacitance, board design issue, or lack of 32 XTAL on board).

@param cal_clk clock to be measured @param slow_clk_cycles number of slow clock cycles to average @return average slow clock period in microseconds, Q13.19 fixed point format, or 0 if calibration has timed out